Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Opry Audio on April 06, 2021, 02:45:15 pm

Title: Transformer Testing
Post by: Opry Audio on April 06, 2021, 02:45:15 pm

Hello! I know there have been many threads here regarding testing unknown transformers. I’ve gotten conflicting info from people about how and what to test, but even when things look ok, it doesn’t always conclude that a transformer isn’t damaged. I have a couple of amps that have smoked and would like to start with determining if the transformers are damaged.

In terms of PT’s. I would assume that a simple ohm measurement of the primary would tell you a lot about if it’s shorted or not. But under v assuming it’s not shorted— when testing the secondary I have been told that putting the meter in parallel with each winding will give you an accurate reading of the total voltage produced. On a 6.3v winding, putting the meter in parallel will show you 6.3v.. or not. Is this also true for the HV winding?

Any advice on testing OT? Can you go through your process of determining if it’s truly undamaged if the amp is not in functional condition?

Title: Re: Transformer Testing
Post by: SILVERGUN on April 06, 2021, 06:40:30 pm

Try this for the PTs:
http://www.geofex.com/ampdbug/pwrtrans.htm (http://www.geofex.com/ampdbug/pwrtrans.htm)

Any good?

And this one for OTs is super secret so dont tell anyone I told you about it.
https://www.premierguitar.com/the-super-secret-transformer-tester (https://www.premierguitar.com/the-super-secret-transformer-tester)

Title: Re: Transformer Testing
Post by: tubeswell on April 07, 2021, 12:33:34 am

 R.G. Keen sure keeps plenty of good secrets handy.

Title: Re: Transformer Testing
Post by: pdf64 on April 07, 2021, 09:25:59 am

As with checking unknown equipment, unknown PTs should be powered up (secondaries disconnected) via your light bulb limiter. That will detect whether there are (hard) shorted turns in any of the windings, and it seems sensible to use one anyway.

Title: Re: Transformer Testing
Post by: Soulfetish on April 07, 2021, 01:29:36 pm

A resistance test is unreliable for testing for shorted turns. I built a quick prototype neon tester, which is demo'd here:
https://vimeo.com/248259853
It worked out so well that I decided to build one using an aluminum Hammond enclosure that was portable, and would stand up under years of abuse.

Title: Re: Transformer Testing
Post by: Soulfetish on April 07, 2021, 01:30:59 pm

Oh, I meant to ask– Do you have a variac?

Title: Re: Transformer Testing
Post by: Opry Audio on April 07, 2021, 05:41:56 pm

Thanks, those links are great! I haven’t seen either of them before.

And yeah, I do have a variac that I use 100% of the time pretty much. I need to build a lightbulb limiter to be honest.

Title: Re: Transformer Testing
Post by: SILVERGUN on April 07, 2021, 07:17:05 pm

Quote from: Opry Audio on April 07, 2021, 05:41:56 pm

Thanks, those links are great! I haven’t seen either of them before.

Then you'll want to follow the red arrow path to enlightenment.
We have been given the gift of knowledge from all of these elder gentlemen and some times it's easy to look right past it.


Imagine what some of these guys had to go through to learn this stuff. BEFORE THE INTERNET.  :w2:   :dontknow:   :think1: